1. Field of the Invention
The present invention relates to a magnetic field sensor and a magnetic disk apparatus.
2. Description of the Related Art
In a magnetic disk apparatus such as an HDD, it is always required to improve the recording density. With improvement in the recording density, a recording bit size recorded in a magnetic disk is diminished, which weakens a magnetic field. Such being the situation, an improvement in sensitivity is required for a magnetic field sensor (reproduction head) for detecting the signal magnetic field.
A typical magnetic field sensor at the present time has a structure that a giant magnetoresistive element (GMR element) is arranged between a pair of magnetic shields where the GMR element is used in such a manner that a sense current is flowed in the plane thereof (current-in-plane, i.e., CIP type).
On the other hand, it is reported that a GMR element in which a sense current is flowed perpendicular to the plane of the GMR element (current-perpendicular-to-plane, i.e., CPP type) permits a magnetoresistance ratio higher than that obtained by the CIP type GMR element. Likewise, it is reported that a tunneling magnetoresistive element (TMR element) also permits a high magnetoresistance ratio. Under the circumstances, it is being studied to use the CPP type GMR element or the TMR element.
A shield type magnetoresistive head (reproduction head) using a CPP type magnetoresistive element is constructed such that a lower magnetic shield, a nonmagnetic conductive layer, a magnetoresistive element, a nonmagnetic conductive layer, and an upper magnetic shield are formed on a substrate. The magnetoresistive element has a basic structure comprising a magnetization free layer (free layer), a nonmagnetic intermediate layer, a magnetization pinned layer (pinned layer), and an antiferromagnetic layer. In the GMR element, a nonmagnetic conductive material such as Cu is used for the nonmagnetic intermediate layer. In the TMR element, an insulating material such as alumina is used for the nonmagnetic intermediate layer. Each of the lower magnetic shield and the upper magnetic shield is formed of a soft magnetic material and serves as an electrode (a positive electrode or a negative electrode) for flowing a sense current into the magnetoresistive element through the nonmagnetic conductive layer. The lower magnetic shield and the upper magnetic shield are connected to a constant current or constant voltage power source such as a head amplifier, and a change in resistance of the magnetoresistive element is detected as a change in voltage or a change in current.
Conventionally, each nonmagnetic conductive layer arranged between the lower magnetic shield and the magnetoresistive element or between the upper magnetic shield and the magnetoresistive element is formed of a stacked film such as Ta/Au/Ta. Au, which has a high electrical conductivity and is excellent in heat radiation performance, contributes to suppress heat of the element. A metal material having the characteristics similar to those of Au includes Ag and Cu. In order to suppress the heat generated by the element, a stacked structure including a thick Au layer (or a thick Ag or Cu layer) is used for the nonmagnetic conductive layer in any of the positive electrode and the negative electrode.
However, it has been found that, in the CPP type magnetoresistive element, Au, Ag or Cu, which is a metal material forming a nonmagnetic conductive layer, having a high electrical conductivity and exhibiting a high heat radiation performance, is likely to be subjected to electromigration with the flow of the current. The electromigration exhibits directionality, where the migration of Au, Ag or Cu from the nonmagnetic conductive layer for the positive electrode toward the magnetoresistive element poses problems. Particularly, if the magnetoresistive element generates heat with the flow of the current under the condition that the element is exposed to a high temperature, Au, Ag or Cu forming the nonmagnetic conductive layer for the positive electrode diffuses into the magnetoresistive element, with the result that the magnetoresistance ratio is lowered. Also, it is possible for the nonmagnetic conductive layer for the positive electrode to finally break. It should be noted that the electromigration is made prominent with decrease in the thickness of the nonmagnetic conductive layer as the gap between the lower magnetic shield and the upper magnetic shield is decreased with improvement in the recording density.
A technique to alter polarities of the electrodes periodically, which is disclosed in U.S. Pat. No. 5,793,550, may be effective to suppress the electromigration. However, if the polarities of the electrodes are altered periodically, the current magnetic field applied to the free layer in the magnetoresistive element is also altered periodically, with the result that performance for magnetic field detection of the magnetoresistive element is adversely affected. Also, the magnetoresistive element cannot be operated while the polarities of the electrodes are altered, which leads to a poor response. Therefore, it is impractical to employ the particular technique.